1. Technical Field
The present invention relates to time reference sources and, more particularly, integrated time references within a device for use either as a clock reference or calibration master in a master-slave continuous-time filter system.
2. Related Art
Integrated time references are essential in integrated devices and are used for many purposes including synchronization of internal operations, synchronization with buses and external networks among other applications. For example, for a device that communicates over an external synchronized bus, it is important that the device has an internal time reference that it can use to detect and respond to the signals on the bus. Generally, synchronized buses require that all operations happen at specified instants in time. Thus, a device must not only be able to read the synchronized signals being received on the bus, but must also be able to transmit at specified instants in a synchronized manner.
Crystal oscillators have long been used to provide very accurate time keeping functions as result of their steady and predictable response to physical or electrical stimuli. Integrated circuits, however, by their very nature, cannot incorporate an internal crystal oscillator. Accordingly, integrated circuit oscillators are often designed using integrated resistors and capacitors to achieve an RC time constant based oscillation. One problem with this approach, however, is that RC time constants vary significantly according to the tolerance of the resistors and capacitors configured to provide the RC time constants. It is, therefore, difficult to design RC time constant circuits for which their operation is predictable and reliable.
In cases where integrated components are required to achieve accuracy, for example, to enable predictable filter response to be achieved, a more stable external device may be used as a reference. There are times, however, where it is disadvantageous to have a device or integrated circuit be coupled to receive precise frequency signals from an accurate external source.
Accurate internal time keeping is needed, for example, by analog-to-digital converter (ADC) circuits. ADC's are complex analog-to-digital converters that are often used to digitize analog wave forms, for example, voice wave forms, as a part of converting a voice signal to a digital signal that may be manipulated, stored or transmitted over a wireless medium.
The conversion of the voice signal from analog to digital will be most accurate and most reproducible if the sampling occurs at precise and constant measures of time. Moreover, internal synchronized buses, for another example, also require accurate time keeping so that a receiver can properly determine when to latch a data signal. A transmitter must be able to accurately drive a signal on a synchronized bus. Thus, for these and many other reasons, a need exists not only for internal time sources that may be used as a reference signal, but also for accuracy.